The great majority of liquid crystal devices used in notebook personal computers or liquid crystal TV sets are operated in the twisted nematic mode. However, in the twisted nematic mode, the image displayed on a liquid crystal device looks different depending on the viewing direction. To improve the viewing direction dependence, it has been proposed to employ an in-plane switching (IPS) mode in which a voltage is applied to a liquid crystal in a lateral direction, as disclosed for example in Japanese Unexamined Patent Publication Nos. 56-091277 and 6-160878.
The principle of operation in the IPS mode will be described briefly with reference to some drawings. FIGS. 4a and 4b are cross-sectional views illustrating the behavior of a liquid crystal in a liquid crystal panel designed to operate in the IPS mode, wherein FIG. 4a is a cross-sectional view of a cell without an application of voltage and FIG. 4b is a cross-sectional view of the cell under the application of a voltage greater than a threshold value. The plane views of FIGS. 4a and 4b are given in FIGS. 4c and 4d, respectively. In FIG. 4, reference numerals 401 and 409 denote a pair of polarizing plates, 402 and 408 denote a pair of substrates between which a liquid crystal is disposed, 403 denotes a color filter, 404 and 406 denote orientating films, and 405 denotes a liquid crystal molecule drawn in a schematic fashion. Furthermore, reference numeral 410 denotes a pixel electrode, 411 denotes a common electrode disposed in a pixel at a location opposite to the pixel electrode, 412 denotes an image signal line (source line), and 407 denotes an insulating layer for isolating the pixel electrode 410 and the common electrode 411 from each other. In the IPS-mode liquid crystal device, as can be seen from FIG. 4, the pixel electrode and the common electrode for applying a voltage across the liquid crystal are disposed on one substrate at locations apart in a lateral direction. Reference numeral 413 denotes the absorption axis of the lower polarizing plate and 414 denotes the absorption axis of the upper polarizing plate.
Although an active element such as a TFT (thin film transistor) is also disposed, it is not shown in FIG. 4. FIGS 4a and 4b are a cross section taken along line X-X′ of FIG. 5, and FIGS. 4c and 4d are an enlarged plane view illustrating an area surrounded by a broken line in FIG. 5, wherein FIG. 5 illustrates the structure of one pixel. In the specific example shown in FIG. 5, two common electrodes 502 and one pixel electrode 501 are disposed in a lateral direction in one pixel, whereas there may be some other number of common electrodes 502 and pixel electrodes 501 in one pixel. Furthermore, in FIG. 5, reference numeral 503 denotes a scanning signal line (gate line), 504 denotes an image signal line (source line), and 505 denotes a thin film transistor(TFT).
Of the pair of substrates 402 and 408, as shown in FIGS. 4a and 4c, a color filter 403 is formed on the upper substrate 402, and a line-shaped common electrode 411 and pixel electrode 410 are formed on the inner surface of the lower substrate 408. Furthermore, orientating films 404 and 406 for orientating the liquid crystal molecules 405 are formed on the inner surfaces of the respective substrates. A liquid crystal is disposed between the pair of substrates 402 and 408. When no voltage is applied, the liquid crystal molecules 405 are uniformly orientated at a fixed angle (within the range from 0 to 45) with respect to the longitudinal direction of the line-shaped electrodes (common electrode 411, pixel electrode 410). In the specific example shown in FIG. 4, the angle is set to 30°. On both sides of the liquid cell, there are disposed polarizing plates 401 and 409. The upper polarizing plate 401 is disposed such that its absorption axis 414 becomes parallel to the orientation of the liquid crystal. On the other hand, the lower polarizing plate 409 is disposed such that its absorption axis 414 becomes perpendicular to the orientation of the liquid crystal. In this state, black is displayed in the pixel. The liquid crystal is made up of a material having positive dielectric anisotropy.
If an electric field 415 is applied, the liquid crystal molecules 405 are aligned so that their longitudinal axis is directed in a direction parallel to the electric field 415, as shown in FIGS. 4b and 4d. As a result, the orientation of the liquid crystal molecules 405 come to have a certain angle with respect to the absorption axis of the polarizing plates. The birefringence of the liquid crystal varies in accordance with the orientation angle of liquid crystal molecules which varies in response to the strength of the applied electric field. Thus, it is possible to control the transmission of light through the pair of polarizing plates thereby controlling the brightness.
In this structure, however, the pixel electrode 410 and the common electrode 411 used to apply a voltage across the liquid crystal are formed on only one substrate and there is no electrode on the other-side substrate. This can cause a problem in that the substrate tends to be electrostatically charged. The electrostatic charge disturbs the orientation of the liquid crystal and thus it becomes impossible to display a high-quality image. Once the substrate is electrostatically charged, it is difficult to remove the electrostatic charge because there is no electrode on the other-side substrate.
In view of the above, it is an object of the present invention to provide a liquid crystal device capable of displaying a high-quality image without being electrostatically charged or without being influenced by an electrostatic charge.